Modified frequency‐wavenumber algorithm for regional seismograms using Filon's quadrature: modelling of Lg waves in eastern North America

Abstract

The main focus of this study is to develop an efficient frequency‐wavenumber code to synthesize high‐frequency (say, 10 Hz) regional seismograms (up to a distance of more than 1000 km) in a medium consisting of a large number of crustal layers, and investigate effects of regional waveguides on Lg waves. In this newly developed code, the frequency‐wavenumber response is evaluated by using the Filon's quadrature at high frequency and the polynomial approximation of Bessei's functions (implementing trapezoidal integration) at low frequency. This has reduced the computation time by several folds compared to the time needed for the trapezoidal integration. Using this algorithm, both broad‐band seismograms at Harvard and short‐period ECTN (Eastern Canadian Telemetered Network) seismograms from 1988 November 25 Saguenay earthquake were successfully modelled. A moment magnitude of 5.9 was used. Initially an average waveguide was calibrated to successfully excite the regional Pnl and Snl waves at HRV, its near‐surface structure constrained by matching the recorded frequency content. The crust‐mantle transition zone was constrained by modelling the strength of the broad‐band Snl waves, consisting of Sn, sSmSnSmSSmS, and sSmSSmS phases, relative to the initial Pnl waves. The model was modified to consist of thin layers with an alternating high‐and low‐velocity distribution to match the high‐frequency ECTN Lg waves observed from 300 to 600 km away. In addition, the effect of a near‐surface irregular receiver structure on the duration of Lg waves was also investigated by convolving its response with the regional Lg seismograms. Initial study suggests this method as viable for modelling the entire Lg waves including the coda. Numerical studies suggest that the shapes and peak amplitudes on Lg seismograms depend more strongly on the source depth, anelasticity and crustal waveguide than do the Pnl seismograms. Copyright © 1994, Wiley Blackwell. All rights reserved